The "cut", represented as a !, is a special mechanism which affects how prolog backtracks.
Examples
?- repeat, !.
yes;
no print_first_sentence(X) :-
atom_chars(X, Chars), member(Next, Chars), write(Next), Next=='.', !.
?- print_first_sentence('word1 word2 word3. word4 word5 word6.').
word1 word2 word3.
yes a(x, Y) :- Y = 1, !.
a(X, Y) :- Y = 2.
?- a(x, Y).
Y = 1
yes
?- a(y, Y).
Y = 2
yes
?- a(z, Y).
Y = 2
yes